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HEAT  THEATMENT.                                    305
attempt to estimate the structure of the rail from the amount of shrinkage is simply putting the cart before the horse; it is much like the practice in vogue a few years ago of rolling octagon spring steel and then defacing the bar by hitting it with a hammer to make it resemble the bars turned out by the tilting hammer. This tilting consisted in. a rapid succession of blows continued during the cooling of the piece until a very low temperature was reached, and by this means the crystalline structure was rendered very fine and the steel was in the very best condition. The rolls did not finish the bar as cold, nor did the effect of rolling penetrate as thoroughly as the blow of the hammer, and this lack could hardly be atoned for by duplicating an incidental accompanying condition. .
There will always be some difference between the structure of the center of the head of the rail and the portion near the surface, but if the rail is rolled at a proper temperature during the passes when considerable work is put upon the piece, this difference will not be serious. No. 25, in Fig. XV-B, shows the center of the head of a girder or tram rail weighing 107 pounds per yard, and No. 26 shows the surface of the head. No. 27 shows the center of the head of a 90-pound girder rail and 'No. 28 the surface. No. 29 is the center of a 70-pound T rail and No. 30 the surface. All these were rolled at Steelton on regular orders and it will be noted that while there is a difference, the structure of the center is very good.
Fig. XV-F shows the structure of T rails rolled at Sparrow's Point at the works of the Maryland Steel Company and represents the best modern practice. No. 31 is the center of a 100-pound T rail and No. 32 the surface; No. 33 the center of an 85-pound T rails, these structures representing the regular practice at the works. Nos. 34 and 35 have already been discussed as hot-rolled and cold-rolled rails. No. 36 represents the structure of a small test bar of rail steel which was rolled for the purpose of this experiment as cold as the strength of the rolls would allow, the finishing temperature being 490° 0. (915° F.), which is considerably below the critical point, as shown by the lines of work appearing in the photograph. This evidently is the finest structure obtainable, and it may be used as a standard by which to estimate the condition of the other pieces. All the photographs in this rail steel series are cross-sections that are magnified forty-six diameters.
SEC. XVm.—Effect of heat treatment upon the structure of cast-